Abstract: According to the geological survey information of Songshunangou diggings in Qinghai province，the computation model of slope in southwest was built. The safety factor was solved by shear strength reduction method.The result under different reduction coefficient was obtained by using the FLAC3D. And the criterion with main plastic zone analysis and assisting by maximum horizontal displacement is determined to achieve the safety factor quickly. After analyzing the computation result，it concludes that the safety factor of southwestern slope is 1.85 under natural state. Through comparative analysis，the fault in southwest slope has a great influence on stability of mine slope，which must be majorly monitored during the mine excavation.

Key words: Safety factor, Open pit mining slope, Shear strength reduction method, Numerical simulation